According to usage and battery capacity, secondary batteries are classified as low-capacity secondary batteries, often referred to as “small batteries,” which use a single or several battery cells packaged in the form of a pack, and high-capacity batteries, often referred to as “large batteries,” which use scores of battery cells packaged into a battery pack.
Small batteries are used as the power source for small electronic devices, such as mobile phones, laptop computers, and camcorders, while large batteries are used as the power source for driving motors in hybrid vehicles and the like.
Small batteries are commonly formed as a prismatic or cylindrical shape when formed with a single cell, and are fabricated by inserting an electrode assembly into a container with a predetermined shape after forming the electrode assembly as a jelly-roll by spirally winding positive and negative electrodes with a separator interposed as an insulator therebetween.
The above positive and negative electrodes have lead terminals, that is, conductive tabs, to collect a current generated therefrom when the battery is operating. In further detail, these conductive tabs are fixed onto the electrode assembly by welding to induce the current generated from the positive and negative electrodes to each positive and negative terminal.
However, when this structure of small batteries is directly applied to large batteries, the operation characteristics of the large batteries are not fulfilled with respect to electric capacity and power. Therefore, a multi-tab structure for attaching a plurality of tabs to an electrode assembly has been suggested in Japanese Patent Laid-Open No. 2003-7346. Here, secondary batteries have a structure where a plurality of tabs formed in one direction of the electrode assembly are combined with an internal terminal, which is connected to an external terminal.
These tabs of a secondary battery are formed as a single body with a current collector or are welded to the current collector after being fabricated separately. However, both structures have their own problems. For example, the tabs formed as a part of a current collector can have difficulties in satisfying the power requirements for large batteries because the material of the current collector tends to be over-consumed. The separately-fabricated tabs also have a disadvantage of needing many working processes. Especially, since the heat generated from the tabs during the welding can have an influence on an electrode assembly, the tabs need to be prepared relatively longer to minimize that influence. Accordingly, the space occupied by the tabs also becomes larger, decreasing the energy density in secondary batteries.
In addition, large batteries used as packs for driving motors require high power characteristics to fulfill their usage. Therefore, tabs need to be designed to minimize the resistances produced during the current collecting process. However, the aforementioned solutions proposed by the conventional art tend to incur relatively increased resistances since they perform current collecting through tabs with small unit areas, resulting in decreased current-collecting efficiency.